In chemistry and biotech it is common to prepare solutions or suspensions and store them for considerable periods of time. This is commonly done, for example, using the wells of a well plate or in plastic tubes placed into a tube holder. Because solutions and suspensions are subject to processes such as solvent evaporation or hydration which may change both their volume and composition over time, it is often useful to periodically determine the height and/or volume of the fluid in a reservoir.
It can also be desired to determine the height of a fluid in a reservoir in order to eject droplets from it. When acoustic ejection is employed (as described for example in U.S. Pat. No. 6,666,541), it is desirable that the focal point of the acoustic energy be near the surface of the fluid. It is therefore desirable for the person or machine controlling the ejection process to know the height of the fluid in the reservoir.
In U.S. Pat. No. 6,938,995, techniques are disclosed for using focused acoustic energy to determine the height and/or volume of the fluid in a reservoir. A toneburst of focused acoustic energy is applied to the reservoir from the outside (commonly from below). The reflections of the focused acoustic energy pulse are detected. Based on the time between the pulse transmission and reflections the height can in many cases be computed. A simple way to compute the height is to multiply the speed of sound in the fluid by one half the time between receipt of an echo from the top of the bottom of the reservoir and receipt of an echo from the fluid surface.
It is known that the free surface of a fluid is not necessarily flat, but can have varying shapes. Commonly the free surface has a somewhat concave shape often called a “meniscus.” The surface is approximately flat and horizontal at the center of the container and tilted toward the edges of the container. Existing techniques for determining the height and/or volume of a reservoir work reasonably well with these kinds of free surfaces. In the case of liquid metals (e.g., mercury at room temperature) or polymeric solutions (e.g., polyisobutylene) in a low shear flow, the free surface of the fluid often has a convex shape.
FIG. 1 illustrates schematically a known arrangement for using focused acoustic energy to measure fluid height in a reservoir such as 10. An acoustic transducer with a focusing means 14 produces focused acoustic energy 12. The energy travels towards the surface of the fluid in reservoir 10. (In normal use an acoustic coupling medium, e.g., water, would occupy the space between the transducer 14 and the bottom of the reservoir 10.) Some of the acoustic energy reflects off the surface of the fluid in reservoir 10 and returns to the transducer 14, where it is sensed.
FIG. 2 depicts a typical waveform that is received at the transducer 14 when the outgoing focused acoustic energy is a pulse-like waveform of a few cycles. As may be seen, there are three distinguishable echoes, first BB from the bottom of the bottom of the reservoir, then TB from the top of the bottom of the reservoir, and then SR which is the reflection from the surface of the fluid in reservoir 10. From the graph one can determine the time difference between the reflections from the bottom of the reservoir and the surface of the fluid. With knowledge of the speed of sound in the fluid in the reservoir, it is straightforward to calculate the additional distance which the echo from the surface of the fluid traveled compared to the echo from the top of the bottom of the reservoir, i.e., the height of the fluid in the reservoir. The computation may readily be automated using a microprocessor or personal computer.
There is a need, however, for methods of determining free surface height which work well for surfaces with more unusual shapes.